Well, they should be "AC grounded" which means that Vref should be a low impedance voltage source. Probably it would be fine to use two equal value resistors and slap a large cap to ground from their junction, like you see with most single supply op-amp circuits. A buffered Vref would be better of course, but maybe overkill.
The schematic also implies that the input signal is referenced to/riding on this same voltage. If the input is referenced to some other voltage, you should include a DC blocking cap in series with the input resistor.
It's worth mentioning that there is actually an "AC ground" in the discrete wah circuit as well. The 4.7uF cap on one end of the inductor is a low impedance for signal currents and its voltage remains more or less constant. It's not as obvious as the "two resistors, one capacitor" reference you might be used to, but it serves the same purpose.